Radomes are structures used on aircraft for covering instruments such as radar devices that transmit and receive electromagnetic radiation to protect such devices from the elements. A radome is constructed to be substantially transparent to the electromagnetic radiation. Conventionally, radomes have been constructed with a honeycomb core covered on its opposite sides by facesheets of fiber-matrix composite material. A drawback of such construction is that under some circumstances water can penetrate the facesheet, such as through a fastener hole or through a defect in the facesheet, and collect inside the honeycomb core. The water trapped in the honeycomb core can then cause further damage through various mechanisms including expansion caused by freezing. Additionally, the water absorbs the electromagnetic radiation, thereby degrading the signal quality and reducing effectiveness of the radar device. Epoxy bond lines also can be degraded by prolonged exposure to the water, which can cause delamination and even failure of the radome. Furthermore, when water collects in the honeycomb core it adds significantly to the weight of the radome, which is particularly undesirable in an aircraft installation.
Foam-core radomes have been developed at least in part as a response to problems such as those mentioned above. Foam is advantageous as a core material in that foam has a reduced tendency to harbor water compared with honeycomb. However, foams that have been used in prior foam-core radomes have less rigidity than the types of honeycomb materials that have traditionally been used. Conventional foam-core radomes are made from polyurethane foam and were popular in the 1950's. But polyurethane foam has a tendency to crumble as well as having poor fatigue and impact properties, and improved foam-core radomes have been largely ignored. Another problem with foam-core radomes is that they require an expensive and time-consuming fabrication process involving an autoclave process.
One method of making a foam-core radome is disclosed by U.S. Pat. No. 5,323,170. The radome includes a closed-cell foam core sold under the trademark Divinycell™ and is surrounded by layers of fiber-reinforced plastic or so-called “prepreg.” The prepreg layers are synthetic fibers that are impregnated with resin and then stored in a cool environment until applied about the core. The completed mold is then cured either in an autoclave or by a difficult multiple-step vacuum and heating process.
Thus, there is a need to provide an improved method for making a foam-core radome that solves the inefficiencies and cost opportunities of the prior art. In particular, there is a need to avoid the time consuming and messy application of prepreg layers, as well as avoiding costly and time-consuming autoclave procedures.